Sodium Ions Cannot Diffuse Passively Through The Plasma Membrane

1. Due to their charged nature, sodium ions cannot diffuse passively through the plasma membrane, which is a lipid bilayer. The sodium ion and its cloud of polarized water are capable of interacting with the polar hydrophilic heads of the phospholipids but not with the hydrophobic tails, so they could only cross the plasma membrane though ions channels, integral membrane proteins that form ion-conducting pores in the lipid bilayer, or pumps. The size of the pore and they way it interacts with ions, gives the channel its ion selectivity, so they allow only one ion to pass through. The sodium ion channels are voltage gated channels, meaning they response to voltage changes across the membrane. The ion channel detects an electric stimulus, it opens to the extracellular space allowing sodium ions to pass though the membrane by simple diffusion. Ion channels allow ions to move passively, no ATP is needed for their transport; however ATP is used to open and close the channels.

The sodium ion channel is a single polypeptide chain made up of more than 1800 amino acids. It’s comprised of 4 domains and each domain has 6 transmembrane a helices segments, (S1-S6) contiguous with each other, folded into a cluster with the channel pore being in the center. The S4 segment is known to act as a voltage sensor. Both the N-terminus and C-terminus are inside the cytoplasm. The N-terminus protrudes into the cytosol and forms an inactivating particle that is on the inside of the cell. The inactivation particle sticks on the channel acting like a plug that prevents the ions from constantly moving though, so the inactivating particle only gets removed when there’s a change in